Atomization and spray characteristics of bioethanol and bioethanol blended gasoline fuel injected through a direct injection gasoline injector

The focus of this study was to investigate the spray characteristics and atomization performance of gasoline fuel (G100), bioethanol fuel (E100), and bioethanol blended gasoline fuel (E85) in a direct injection gasoline injector in a gasoline engine. The overall spray and atomization characteristics such as an axial spray tip penetration, spray width, and overall SMD were measured experimentally and predicted by using KIVA-3V code.The development process and the appearance timing of the vortices in the test fuels were very similar. In addition, the numerical results accurately described the experimentally observed spray development pattern and shape, the beginning position of the vortex, and the spray breakup on the spray surface. Moreover, the increased injection pressure induced the occurrence of a clear circular shape in the downstream spray and a uniform mixture between the injected spray droplets and ambient air. The axial spray tip penetrations of the test fuels were similar, while the spray width and spray cone angle of E100 were slightly larger than the other fuels. In terms of atomization performance, the E100 fuel among the tested fuels had the largest droplet size because E100 has a high kinematic viscosity and surface tension.     

Experimental and numerical analysis of spray-atomization characteristics of biodiesel fuel in various fuel and ambient temperatures conditions

The purpose of this work is to reveal the effects of fuel temperatures and ambient gas conditions on the spray-atomization behavior of soybean oil methyl ester (SME) fuel. The spray-atomization behavior was analyzed through spray parameters such as the axial distance from the nozzle tip, local and overall Sauter mean diameter (SMD). These parameters were obtained from a spray visualization system and a droplet measuring system. In addition, the experimental results were compared with the numerical results calculated by the KIVA-3V code. It was revealed that the increase of the fuel temperature (from 300 K to 360 K) little affects the spray liquid tip penetration. The increase of the ambient gas temperature (from 300 K to 450 K) caused a increase in the spray liquid tip penetration. Also, biodiesel fuel evaporation actively occurred due to the increase in the fuel temperature and the ambient gas temperature. Of special significance was that the highest vapor fuel mass concentration was observed at the center region of the spray axis. In the results of the microscopic characteristics, the detected local droplet size at the axial direction and overall droplet size at the axial and radial direction in a control volume increased when the fuel temperature increased. This is believed to be due to an increase in the number of small droplets that quickly evaporated. In addition, the increased fuel temperature caused the decrease of the number of droplets and the increase of the vapor fuel mass. The mean axial velocity of droplets decreased with increasing fuel temperature.     

Flow and atomization characteristics of a twin-fluid nozzle with internal swirling and self-priming effects

A twin-fluid nozzle was proposed for low-pressure atomization. The nozzle is featured by swirling air flows in the mixing chamber. Liquid medium is thereby inhaled due to the pressure difference. An experimental work was performed to investigate the atomization performance of the nozzle and the hydrogen peroxide solution served as the liquid medium. Droplet size and droplet velocity were measured. Effects of the diameter of the air-injection orifice and the air-injection pressure were investigated. The results show that small droplet size is achieved with the proposed nozzle. As the spray develops, Sauter mean diameter (SMD) of the droplets decreases first and then increases, irrespective of the variation of the air-injection orifice diameter and the air-injection pressure. Overall SMD varies inversely with the air-injection orifice diameter and air-injection pressure. Near the nozzle, cross-sectional velocity distribution exhibits a peak-valley pattern, which is replaced with uniformized velocity distributions away from the nozzle. Similarity of cross-sectional radial velocity distribution at different air pressures is evidenced. Furthermore, the correlation between droplet size and droplet velocity is established.     

Large eddy simulation of high pressure spray with the focus on injection pressure

This paper presents a detailed numerical analysis of diesel engine spray structure induced by the Engine Combustion Network (ECN) Spray A at different injection pressures. The non-reacting simulations are performed using OpenFOAM where an Eulerian–Lagrangian model is adopted in the large eddy simulation (LES) framework. Effects of the LES mesh resolution as well as the spray model parameters are investigated with the focus on their impact on spray structure as the injection pressure varies. The predicted liquid and vapour penetration lengths agree well with the measurements at different injection pressures. The mixture fraction is well captured for the injection pressure of 100 and 150 MPa while a slight deviation from the measurements is observed for the injection pressure of 50 MPa near the nozzle. The parametric analysis confirms that the LES mesh resolution has significant effects on the results. A coarser mesh leads to higher liquid and vapour penetration lengths where the deviation from the measurements is larger, resulting in the highest error at the lowest injection pressure. As the mesh size increases, the droplet size distribution becomes narrower, its pick moves to the smaller droplet size and the probability of droplets with higher temperature increases. On the other hand, with increasing the mesh size, the carrier gas velocity decays slower and its radial dispersion decreases. It is found that the droplet characteristics are more affected by the mesh resolution when the injection pressure is the lowest while the opposite is true for the carrier phase. The number of Lagrangian particles also affects the droplet characteristics and the fuel-air mixing but their effects are not as significant as the mesh size. The results become less sensitive to the number of Lagrangian particles as the pressure injection decreases. Finally, the importance of the initial droplet size distribution is investigated, confirming its impact is marginal, particularly on the liquid length. It is observed that the initial droplet size is only important at very close to the nozzle and its impact on the spray structure becomes quickly insignificant due to the high rates of breakup and evaporation. This trend is consistent at different injection pressures.     

Macroscopic and microscopic characteristics of a fuel spray impinged on the wall

An experimental study was performed to investigate the macroscopic behavior and atomization characteristics of a high-speed diesel spray impinged on the wall at various injection and impinging conditions. The development processes of sprays impinged on the wall were visualized using the spray visualization system composed of a Nd:YAG laser and an intensified charge-coupled device (ICCD) camera. The atomization characteristics of the impinged spray on the wall were also explored in terms of mean droplet diameter and velocity distributions by using a phase Doppler particle analyzer (PDPA) system. The results provide the effects of injection parameters, wall conditions, and the other various experimental conditions on the macroscopic behavior and atomization characteristics of the impinged sprays on the wall.     

Dynamic characteristics of pulsed supersonic fuel sprays

This paper describes the dynamic characteristics of pulsed, supersonic liquid fuel sprays or jets injected into ambient air. Simple, single hole nozzles were employed with the nozzle sac geometries being varied. Different fuel types, diesel fuel, bio-diesel, kerosene, and gasoline were used to determine the effects of fuel properties on the spray characteristics. A vertical two-stage light gas gun was employed as a projectile launcher to provide a high velocity impact to produce the liquid jet. The injection pressure was around 0.88–1.24 GPa in all cases. The pulsed, supersonic fuel sprays were visualized by using a high-speed video camera and shadowgraph method. The spray tip penetration and velocity attenuation and other characteristics were examined and are described here. An instantaneous spray tip velocity of 1,542 m/s (Mach number 4.52) was obtained. However, this spray tip velocity can be sustained for only a very short period (a few microseconds). It then attenuates very quickly. The phenomenon of multiple high frequency spray pulses generated by a single shot impact and the changed in the angle of the shock structure during the spray flight, which had already been observed in previous studies, is again noted. Multiple shock waves from the conical nozzle spray were also clearly captured.      

A unified spray model for engine spray simulation using dynamic mesh refinement

This study is based on dynamic mesh refinement and uses spray breakup models to simulate engine spray dynamics. It is known that the Lagrangian discrete particle technique for spray modeling is sensitive to gird resolution. An adequate spatial resolution in the spray region is necessary to account for the momentum and energy coupling between the gas and liquid phases. This study uses a dynamic mesh refinement algorithm that is adaptive to spray particles to increase the accuracy of spray modeling. On the other hand, the accurate prediction of the spray structure and drop vaporization requires accurate physical models to simulate fuel injection and spray breakup. The present primary jet breakup model predicts the initial breakup of the liquid jet due to the surface instability to generate droplets. A secondary breakup model is then responsible for further breakup of these droplets. The secondary breakup model considers the growth of the unstable waves that are formed on the droplet surface due to the aerodynamic force. The simulation results are compared with experimental data in gasoline spray structure and liquid penetration length. Validations are also performed by comparing the liquid length of a vaporizing diesel spray and its variations with different parameters including the orifice diameter, injection pressure, and ambient gas temperature and density. The model is also applied to simulate a direct-injection gasoline engine with a realistic geometry. The present spray model with dynamic mesh refinement algorithm is shown to predict the spray structure and liquid penetration accurately with reasonable computational cost.     

Atomized non-equilibrium two-phase flow in mesochannels: Momentum analysis

Two-phase pressure drop measurements are very difficult to make while the fluid is in non-equilibrium condition, i.e. while phase change is taking place. This is further complicated when an atomized liquid is introduced in the system at much higher velocity than other components such as liquid layer, vapor core, and entrained droplets. The purpose of this paper is to develop a model to predict the two-phase pressure characteristics in a mesochannel under various heat flux and liquid atomization conditions. This model includes the momentum effects of liquid droplets from entrainment and atomization. To verify the model, an in-house experimental setup consisting of a series of converging mesochannels, an atomization facility and a heat source was developed. The two-phase pressure of boiling PF5050 was measured along the wall of a mesochannel. The one-dimensional model shows good agreement with the experimental data. The effects of channel wall angle, droplet velocity and spray mass fraction on two-phase pressure characteristics are predicted. Numerical results show that an optimal spray cooling unit can be designed by optimizing channel wall angle and droplet velocity.     

Numerical research of diesel spray and atomization coupled cavitation by Large Eddy Simulation (LES) under high injection pressure

A special spray model is applied to study the spray behavior with high injection pressure and micro-hole nozzle. To reveal the cavitation in diesel nozzle and its influence on spray and atomization, the Large Eddy Simulation (LES) turbulence model is adopted to detect the cavitation, and then the special spray model coupling the cavitation is build. From research results, three important conclusions can be drawn. Firstly, the cavitation flow can raise the effective velocity at the nozzle exit and such effect become even more obvious with higher injection pressure, e.g.180 MPa. Secondly, the applied spray model is in good agreement with the spray characteristics and images obtained from the EFS8400 spray test platform. Thirdly, the cavitation with high injection pressure and micro-hole nozzle can increase the spray cone angle and reduce the spray penetration; the cavitation intensity has a great impact on the spray velocity field and vorticity intensity, especially at the initial spray field under the condition of high injection pressure.     

阶梯型加速段对旋流喷嘴雾化特性的影响

旋流内芯是压力旋流式喷嘴最主要的旋流发生构件, 其几何特征直接影响压力旋流式喷嘴的喷雾特性.目前采用平滑型加速段的旋流内芯导流效率较低.为减小高流量条件下的能量损失, 使喷嘴旋流内芯加速段对喷雾介质产生预旋效应, 增强旋流强度, 本文设计喷嘴旋流内芯加速段为阶梯型, 其下段阶梯相对上段阶梯旋转15°, 旋向与喷嘴旋流槽方向相同.利用粒子动态分析仪(particle dynamics analysis system, PDA) 和高速摄影(charge coupled device, CCD)系统实验研究了加速段结构改进前后喷嘴的喷雾流量、雾场索特尔平均直径(Sauter mean diameter, SMD)、雾滴速度以及喷雾锥角, 并分析了SMD、 雾滴速度的轴向和径向分布特性. 结果表明, 背压差0.08\mathrm{~} 0.46 MPa 范围内, 阶梯型加速段对喷雾介质具有较好的预旋效果.喷嘴的流量提高了48.0% ~ 51.8%; 喷雾的轴向速度提升了31.4% ~ 32.8%, 径向速度提升了1.6% ~ 16.8%; 喷雾锥角减小了4.21°~6.57°; 较高背压差下喷雾下游的SMD减小了9.8%.与平滑型加速段相比, 阶梯型加速段的设计有效地提高了喷嘴的雾化质量.      

Experimental and Numerical Simulations of Spray Impingement and Combustion Characteristics in Gasoline Direct Injection Engines under Variable Driving Conditions

Gasoline direct injection (GDI) increases engine power output and reduces emissions. In GDI engines, increasing injection pressure improves atomization, which increases thermal efficiency at the cost of wall wetting. When wall wetting occurs, both soot emissions and fuel consumption increase. Wall wetting in GDI engines under cold driving conditions has rarely been considered. In this study, experimental data characterizing droplet splashing/spreading phenomena were collected to inform numerical simulations of combustion characteristics and wall wetting subject to variable driving conditions and excess air ratio, λ. Fully 3D and unsteady numerical simulations were carried out to predict flow-field, combustion, and spray-impingement characteristics. To simulate a GDI engine, a spray-impingement model was developed using both experimental data and previous modeling efforts. The excess air ratio and driving-condition temperature were the variable parameters considered in this study. When decreasing λ from 1.0 to 0.7 by increasing the fuel-injection rate (fuel rich), the cylinder pressure increases to 61 % of the pressure when λ=1.0. Because of increasing the fuel-injection rate, the increased momentum in the fuel spray increases both wall wetting and soot generation. At low driving-condition temperatures, the cylinder pressure was up to 63 % less than that under warm conditions, but with increased soot generation. Simulations revealed a correlation between wall wetting and the soot emissions. Soot generation was most sensitive to changes in wall wetting.     

在轴向气流作用下液体轴对称抛撒二次破碎的实验研究

液体轴对称抛撒的实验研究是以云雾爆轰武器的研制为背景的。为了研究轴向气流作用下液体轴对称抛撒二次破碎所形成的雾化场特性,本文利用两台激波管并对之加以改造,成功地在实验室实现了轴向气流作用下液体的轴对称抛撒。为了研究其雾化场的远场特性,本文利用激光粒子测量仪获得了在不同实验工况和不同位置下的雾化场SMD分布曲线。实验数据表明,由于轴向气流速度的增加,液体破碎的Weber数得到了提高,导致二次破碎初期雾化场的SMD随之减小;随着抛撒驱动压力的提高,二次破碎初期雾化场的SMD也随之减小;在同一工况下,雾化场SMD随着测量位置与喷口距离的增加而变大。     

Droplet dynamics and size characterization of high-velocity airblast atomization

Airblast atomizers are especially useful and commonplace in liquid fuel combustion applications. However, the spray formation processes, the droplet dynamics and the final drop size distributions are still not sufficiently understood due to the coupled gas-liquid interactions and turbulence generation. Therefore, empirical and semi-empirical approaches are typically used to estimate the global spray parameters. To develop a physical understanding of the spray evolution, a plain-jet airblast atomizer was investigated in an atmospheric spray rig using the phase-Doppler technique. The simultaneous drop size and axial and radial velocity components were measured on radial traverses across the spray at various axial distances from the nozzle for a range of atomizing pressures. The droplet turbulent and mean kinetic energies were found to be proportional to the atomizing pressure. Hence, the scatter of the radial motion of the droplets increased with the atomizing pressure. A droplet stability analysis was performed to locate the regions characterized by ongoing secondary atomization. The volume-to-surface diameter, D₃₂, of the fully developed spray was compared with estimates provided by five published formulae. The role of liquid viscosity, hence the Ohnesorge number, was found to be negligible in the investigated regime. Three commonly used size distribution functions were fitted to the measured data to analyze their dependence on the atomizing pressure. The Gamma distribution function was found to give the best approximation to the atomization process.     

Nonlinear modeling of drop size distributions produced by pressure-swirl atomizers

An axisymmetric boundary element method (BEM) has been developed to simulate atomization processes in a pressure-swirl atomizer. Annular ligaments are pinched from the parent sheet and presumed to breakup via the linear stability model due to Ponstein. Corrections to Ponstein’s result are used to predict satellite droplet sizes formed during this process. The implementation provides a first-principles capability to simulate drop size distributions for low viscosity fluids. Results show reasonable agreement with measured droplet size distributions and the predicted SMD is 30–40% smaller than experiment. The model predicts a large number of very small droplets that cannot typically be resolved in an experimental observation of the spray. A quasi-3-D spray visualization is presented by tracking droplets in a Lagrangian fashion from their formation point within the ring-shaped ligaments. A complete simulation is provided for a case generating over 80,000 drops.     

Transient analysis of intermittent multijet sprays

This paper analyzes the transient characteristics of intermittent sprays produced by the single-point impact of multiple cylindrical jets. The aim is to perform a transient analysis of the intermittent atomization process to study the effect of varying the number of impinging jets in the hydrodynamic mechanisms of droplet formation. The results evidence that hydrodynamic mechanisms underlying the physics of ligament fragmentation in 2-impinging jets sprays also apply to sprays produced with more than 2 jets during the main period of injection. Ligaments detaching from the liquid sheet, as well as from its bounding rim, have been identified and associated with distinct droplet clusters, which become more evident as the number of impinging jets increases. Droplets produced by detached ligaments constitute the main spray, and their axial velocity becomes more uniformly distributed with 4-impinging jets because of a delayed ligament fragmentation. Multijet spray dispersion patterns are geometric depending on the number of impinging jets. Finally, an analysis on the Weber number of droplets suggests that multijet sprays are more likely to deposit on interposed surfaces, thus becoming a promising and competitive atomization solution for improving spray cooling.     

Large eddy simulation of fuel injection and mixing process in a diesel engine

The large eddy simulation(LES) approach implemented in the KIVA-3V code and based on one-equation sub-grid turbulent kinetic energy model are employed for numerical computation of diesel sprays in a constant volume vessel and in a Caterpillar 3400 series diesel engine.Computational results are compared with those obtained by an RANS(RNG k-ε) model as well as with experimental data.The sensitivity of the LES results to mesh resolution is also discussed.The results show that LES generally provides flow and spray characteristics in better agreement with experimental data than RANS;and that small-scale random vortical structures of the in-cylinder turbulent spray field can be captured by LES.Furthermore,the penetrations of fuel droplets and vapors calculated by LES are larger than the RANS result,and the sub-grid turbulent kinetic energy and sub-grid turbulent viscosity provided by the LES model are evidently less than those calculated by the RANS model.Finally,it is found that the initial swirl significantly affects the spray penetration and the distribution of fuel vapor within the combustion chamber.     

Effect of ambient pressure on penetration of a diesel spray

In the present experimental and theoretical work the propagation of a high-speed fuel spray at distances much longer than the breakup length is studied. The motion of the spray is modeled in two regions: the main region of the steady flow and the front region of the spray. The analysis yields the equation of propagation of the tip of the spray. These theoretical results have been validated against experimental data obtained from a common-rail diesel injection nozzle and from other data available in the literature. The importance of the shock wave propagation at the initial stage of the spray injection is demonstrated.     

Velocity field analysis of the high density,high pressure diesel spray

In this study, particle image velocimetry (PIV) measurements have been performed extensively on a non-reactive dense diesel spray injected from a single orifice injector, under various injection pressure and steady ambient conditions, in a constant flow chamber. Details of PIV setup for diesel spray measurement without additional seeding are explained first. The measured velocity profiles are compared to those obtained from other similar measurements performed in a different institution, as well as those obtained from a 1D spray model simulation, presenting in both cases a good level of agreement. In addition, the velocity fields under various injection pressures and ambient densities show the dominant effects of these parameters on the behavior of diesel spray. The self-similarity of the transverse cut profiles of axial velocity is evaluated, showing that the measurements are in agreement with the hypothesis of self-similar velocity profiles. Finally, the effect of injection pressure and ambient density on the velocity fluctuations is presented and analyzed as well. While the experimental results presented here could help to understand the complex diesel fuel–air mixing process during injection, they also provide additional spray velocity data for future computational model validation, following the main idea of the Engine Combustion Network.     

Numerical and experimental investigation on droplet dynamics and dispersion of a jet engine injector

In the case of turbine combustors operating with liquid fuel the combustion process is governed by the liquid fuel atomization and its dispersion in the combustion chamber. By highly unsteady flow field conditions the transient interaction between the liquid and the gaseous phase is of interest, because it results in a temporal variation of air–fuel ratio which leads to a fluctuating temperature distribution. The objective of this research was the investigation of transient flow field phenomena (e.g. large coherent structures) on droplet dynamics and dispersion of an isothermal flow (of inert water droplets) as a necessary first step towards a full analysis of spray combustion in real-life devices. The advanced injector system for lean jet engine combustors PERM (Partial Evaporated Rapid Mixing) was applied, generating a dilute polydispersed spray in a swirled flow field. Experiments were performed using Phase Doppler Anemometry (PDA) and a patternator to determine the droplet polydispersity, concentration maps, and velocity profiles in the flow. An important finding is the effect of large-scale coherent structures due mainly to the precessing of the vortex core (PVC) of the swirling air jet on the particle dispersion patterns. The experimental results then serve as reference data to assess the accuracy of the Eulerian–Lagrangian computations using a Large Eddy Simulation (LES), a Unsteady Reynolds-Average Navier–Stokes Simulation (URANS) and two simplified (steady-state) simulations. There, a simplified droplet injection model was used and the required boundary conditions of injected droplet sizes were obtained from measurements. Important transient effects of deterministic droplet separation observed during experiments, could be perfectly replicated with this injection model. It is convincingly shown, through extensive computations, that the resolution of instantaneous vortical structures is indeed crucial; hence the LES, or a reasonably-well resolved URANS are preferred over the steady-state solutions with additional, stochastic-type, turbulent dispersion models.